How does mechanical work turn into heat through friction?

In physics, friction acts as a force that resists the motion of objects, effectively degrading mechanical energy by converting it into heat.1

When two surfaces move against each other, friction arises from microscopic irregularities and adhesion between contact regions. The process of overcoming these frictional forces—such as shearing "welded" junctions between metals or deforming materials during rolling—requires work. Because this work is dissipated rather than stored as potential or kinetic energy, it manifests as heat.21

This conversion is a fundamental aspect of the first law of thermodynamics, which dictates the conservation of energy. However, unlike other energy transformations, the conversion of mechanical work into heat is not completely reversible; the second law of thermodynamics establishes that this generated heat can never be converted entirely back into useful mechanical work.1